Water heaters for commercial and home use are well known in the industry. The most common water heaters have a water tank and a series of heat exchange tubes immersed in the water. Hot gasses from the combustion of gas and/or oil are circulated through the tubes, thereby heating the tubes and transferring heat to the surrounding water. These water heaters utilize what is known thermal stacking—hot water moves toward the top of the tank. In such, the heat exchanger is located toward the bottom of the tank in the coolest water to maximize condensing. This type of design requires a tall water heater tank requiring space and not allow for multiple heaters to be stacked. Any mixing of the hot water with the cold or conduction through the tank walls will increase the temperature of the water at the bottom of the tank and reduce condensation and hence, reduce efficiency.
In general, the efficiency of the amount of heat energy delivered to the water from the combustion (hot gasses) is proportional to the difference in temperature between the water and the hot gasses. It is further proportional to the area of the heat exchange tubes—the greater the area, the higher the efficiency. For example, water that is at 55° accepts more heat from gasses that are at a particular temperature than water that is at 95°. As the water heats, more heat from the hot gasses passes out the exhaust system into the atmosphere.
To reduce the amount of wasted heat, multi-stage water heaters have been devised to increase the length, an therefore area, of the exchange tubes. For example, U.S. Pat. No. 4,938,204 to Adams which is hereby incorporated by reference. The disclosed water heater extends the length/area of heat exchange through the use of a second set of heat exchangers. In one embodiment, the second set of heat exchangers are immersed within the same hot water as the first set while in a second embodiment, each is submersed in a separate water tank, the water outflow from the tank with the second set of heat exchangers feeding the water inflow of the other water tank. In this design, the cold water in a first tank is heated by the first set of heat exchangers, and then the exhaust heat from the first set of heat exchangers passes through a second set of heat exchangers immersed within the second tank. The described embodiments have improvements in efficiency over prior water heaters, but requires two large-sized water tanks, both having an outer surface exposed to ambient air, a major factor in energy loss. Additionally, the efficiency of this heater is less than optimal because a percentage of its efficiency is in the form of trapped water vapor that, in this design, is exhausted out the flue as waste along with the other products of combustion. Furthermore, in its two-stage embodiment, two individual tanks are required, stacked one above the other, disallowing stacking in multiple water heater applications. Additionally, the lower tank cannot be used for hot water storage.
What is needed is a high efficiency water heater that effectively transfers as maximum amount of heat from the heat source to the water while reducing losses to the ambient air.